Organic semiconductor polymers have been a subject of active research in recent years in the field of organic electronics, and the polymers are used in organic electroluminescent elements that emit light when electricity is passed, organic photoelectric conversion elements that generate power when irradiated with light, organic thin film transistor elements that control the amount of current or the amount of voltage, and the like. In such an element, similarly to an inorganic semiconductor material, use is made of an organic semiconductor material obtained by combining a p-type conductive semiconductor material, which is an electron donor material, and an n-type conductive semiconductor material, which is an electron acceptor material.
In recent years, since fossil energy of petroleum and the like emit carbon dioxide to the atmosphere, for the purpose of global environment preservation with the suppression of global warming, there is an increasing demand of solar cells. Known examples of organic solar cells that use organic photoelectric conversion elements include a wet type dye-sensitized solar cell (Gräetzel cell) and a total solid type organic thin film solar cell. Since the latter does not use an electrolyte solution, there is no need to take evaporation of this electrolyte solution or liquid leakage into consideration, the solar cell can be made flexible, and the structure of the solar cell or production thereof is more convenient than that of the former.
However, the photoelectric conversion efficiency of organic thin film solar cells is still insufficient. The photoelectric conversion efficiency is calculated by short circuit current (Jsc)×open circuit voltage (Voc)×fill factor (FF); according to which, in order to increase this efficiency, an increase in the open circuit voltage is also needed along with an increase in the short circuit current. The short circuit current is increased when an organic semiconductor material having high solubility and carrier mobility (for example, a compound having a fluorene structure or a silafluorene structure) is used. The open circuit voltage, which is said to be connected with the difference between the HOMO energy level of the p-type conductive semiconductor material and the LUMO energy level of the n-type conductive semiconductor material, is raised when this difference is increased. Furthermore, in the case of an organic solar cell, in order to increase the efficiency, it is efficient to absorb more light of a region of longer wavelengths (650 nm to 800 nm) than the wavelengths of sunlight. Therefore, band gap narrowing is desirable. It is expected that the enhancement of luminescence efficiency, that is, enhancement of the power efficiency of organic electroluminescent lighting for an organic electroluminescent element.
On the other hand, studies on organic semiconductor polymers as p-type conductive semiconductor materials, which are electron donor materials, are in active progress. For example, Patent Literature 1 suggests a polymer having a particular cyclopentadithiazole structure, and Patent Literature 2 suggests a polymer having a particular silacyclopentadithiophene structure. However, in both cases, the conversion efficiency of the solar cell using the above-mentioned polymers is not necessarily sufficient, and these polymers are not satisfactory in terms of durability, and particularly in terms of durability in the presence of oxygen.